The built-in or internal volume of a rotary compressor should maintain a specific ratio relation to the pressure ratio of the compressor if optimum efficiency is to be obtained. It must therefore be possible to vary the internal volume depending on whether full load or partial load prevails. The internal volume can be controlled, for instance, by an axially adjustably slide. Stepwise control can be obtained by using several radially adjustable air valves, for instance. Stepless control of the internal volume, however, usually requires an extremely complicated control system which may comprise a hydraulic system with solenoid valves and some form of calculating unit, e.g. a processor, which controls the solenoid valves depending on the prvailing pressure ratio. The solenoid valves in the hydraulic system are opened or closed to allow different pressure levels access to a hydraulic motor connected to an adjustable slide, for instance, to position this and thus control the internal volume.